Apparatus and process thereof for coating with polytetrafluoroethylene and other materials

ABSTRACT

An apparatus and method thereof are described for coating surfaces with a continuous unitary film of polytetrafluoroethylene (PTFE) by spraying a heated stream of particles from a spray gun. The gun includes a heating means such as an electrical resistance unit, a mixing chamber where gas and preferably inert gas is mixed with the particles, and means for directing a stream of gas through the gun to force the heated particles onto the surface to be coated. Obviously, other plastic materials may be used at their appropriate melting temperatures.

United States Patent Deakin [451 July 18, 1972 [54] APPARATUS ANDPROCESS THEREOF FOR COATING WITH POLYTETRAFLUOROETI'IYLENE AND OTHERMATERIALS [72] Inventor: Stanley Thorns Deakin, Portsmouth, En-

gland [73] Assignee: Sealectro Corporation, Mamaroneck, NY.

[22] Filed: March 1, 1971 [21] Appl.No.: 119,850

Related US. Application Data [62] Division of Ser. No. 507,504, Nov. 12,1965, aban FOREIGN PATENTS OR APPLICATIONS 1,054,610 10/1953 France..239/85 Schleer et al. ..239/ l 35 Primary Examiner-Lloyd L. KingAttorney-James M. Heilman ABSTRACT An apparatus and method thereof aredescribed for coating continuous unitary film of polytetrafluoroethylene(PTFE) by spraying a heated stream of particles from a spray gun. Thegun includes a heating means such as an electrical resistance unit, amixing chamber where gas and preferably inert gas is mixed with theparticles, and means for directing a stream of gas through the gun toforce the heated particles onto the surface to be coated. Obviously,other plastic materials may be used at their appropriate 5 Clains, 7Drawing Figures doned' surfaces with a 521 u.s.c|...239/85,239/8l,239/l33, 239/135 51 Int." B44d3/60 5s FieldoiSeareh..239/142,144,135,133,79-s5 [56] References Cited UNITED STATES PATENTSmeltinstemperawres- 3,415,450 12/1968 Hawk, Sr. "239/85 LLMBJMXH [U 7 ifA 1 g "W LL w X224 "22 J22 Patented July 18, 1972 2 Sheets-Sheet l V mmW ew x Patented July 18, 1972 3,677,471

2 Sheets-Sheet 2 APPARATUS AND PROCESS THERmF FOR COATING WITHPOLY'IEI'RAFLUOROEIHYLENE AND (Yl'HER MATERIAIS RELATED APPLICATIONSThis application is a divisional application divided from Deakinscopending application Ser. No. 507,504now abandoned, filed Nov. 12,1965, in which claimswere elected to the method. This parent applicationisbeing abandoned herewith. This parent application was assignedtoS TROLIMITED, a wholly owned subsidiary of the present assignee, and theassignment was recorded in Reel 1768, Frame 750.

This application is related to stand application Ser. No. 109,369, filedJan. 1971, which is a continuation-in-part of application Ser. No.834,292, abandoned on the same date. This application is also related toStand U.S. Pat. application Serial No. 835,876, filed June 4, 1969,entitled Method of Depositing Heat Fusible Material and ApparatusTherefor, issuing into U.S. Pat..No. 3,591,759, granted July 6, 197l,and to Stand U.S. application Ser. No. 834,293, filed June [8, 1969,titled Spray Nozzle for Plasma Guns, issuing into U.S. Pat. No.3,627,204, granted Dec. I4, 1971, and also to Stand and Streicher U.S.application Ser. No. 868,487, filed Oct. 22, 1969, titled Powder Feeder,issuing into U.S. Pat. No. 3,606,48l, granted Sept. 20, 1971. All theforegoing applications are incorporated by reference herein in theirentirety, and these applications and the present application areassigned to the same arsignee, i.e., SEALECI'RO CORPORA- TION.

This invention relates to the apparatus for coating surfaces (e.g. thesurfaces of metallic articles such as electrical components, bearingsand cooking utensils and surfaces of other materials such as wood) withpolytetrafluoroethylene (PTFE), or other plastics.

It is known to apply coatings of polytetrafluoroethylene in suspensionin a carrier liquid to surfaces and, after drying, to heat the coatedsurfaces to sinter the polytetrafluoroethylene particles to produce acontinuous adherent layer.- The temperature necessary to effectsintering is, however, high (approximately 300 350 C., for example, 325-327 C. or above) and this may have deleterious effects (e.g. softeningor distortion) on the article and is also a costly and inconvenientstep. The present invention provides a coating method which avoids theneed to heat the article and also apparatus for use in carrying out themethod.

The inventionprovides an apparatus and coating of a surface withpolytetrafluoroethylene in which the method comprises the step ofprojecting polytetrafluoroethylene particles through a heating zone,wherein the particles reach sintering temperature, and onto the surfaceto be coated and on which the hot particles coalesce.

Preferably the particles are projected by blowing a stream of gas (e.g.nitrogen, argon or air) through a bed or mass of the particles.

The heating zone may comprise a tubular heating element.

Preferably the heating zone is heated by a source of infrared radiationdesirably within the wavelength range of from 7.5 to 9.0 microns orabove 12.0

In accordance with one aspect of the present invention there is used inthe apparatus and method, particles which have been prepared bycompressing polytetrafluoroethylene (P'I'FE) (e.g. produced byflocculation or coagulation from a suspension produced by suspensionpolymerization), sintering the compressed particles to form a coherentbody and them grinding the body to form fresh particles. The particlesso produced and used in the method may be within the range of sizes from10 microns through 100 microns to 1,000 microns. They may be of aboutthe same size as the particles in the original suspension.

The particles may be produced by grinding (i.e., abrading) from the bodyor by crushing and grinding (e.g. in a ball mill). The grinding may becarried out wet and the particles may be dried before use.

The polytetrafluoroethylene particles may be presintered before beingfed to the heating zone. Presintering has the effect of reducing oreliminating air spaces in the particles. The particles may cake togetherbut the cakes can readily be brokenup before feeding the particles tothe heating zone.

While for convenience and simplicity P'I'FE plastic coating isreferredto throughout, it is understood that any plastic or plastic-like coatingmaterial may be used just as satisfactorily, but that the herein statedtemperatures would have to be adjusted according to the meltingtemperatures of the particular plastic, plasticqlike or combinations ofplastics used. Alternatively powdered polytetrafluoroethylene may beused as supplied by the manufacturers, if it is subjected topre-sintering, e.g. for 7 hours at 500 F. Particles of various sizes maybe used in the methods of the invention but it has been found desirableto employ particles of substantially uniform size in any one coatingoperation.

Pigmented polytetrafluoroethylene may be employed in the method of theinvention but the pigment must have suflicient thermal stability towithstand the temperature of the heating zone.

All kinds of surfaces may be coated by the method of the inventionincluding surfaces, such as wood, which would normally char at thesintering temperature of polytetrafluoroethylene. Surfaces which can becoated include copper, brass, aluminum and steel. The method may be usedto coat skis and sledge runners, tanks and vats used in chemicalprocesses, metal foil and tempered steel parts. Usually surfaces to becoated are first degreased and where appropriate the surface may beetched or sand-blasted to improve adhesion of thepolytetrafluoroethylene.

The particles of polytetrafluoroethylene may be projected at high speedeven up to the speed of sound.

The coating of polytetrafluoroethylene may for example have a thicknessof from 0.001 to 0.100 inch. It is possible to build up a thick coatinggradually by application of two or more coatings one on top of another.In the method of the invention the time during which the particles areheld at sintering temperatures is short and thus less degradation of thepolytetrafluoroethylene will occur leading to better adhesion of thecoating to the surface compared to the normal method of applying a layerof unsintered polytetrafluoroethylene and sintering the layer on thearticle. The coating produced by the method of the invention willusually have less porosity than a coating produced by conventionalmethods such as the one just mentioned.

Various methods of feeding the particles of polytetrafluoroethylene tothe heating zone may be used in the present invention. According to afeature of the present invention, the particles ofpolytetrafluoroethylene are fed by mechanical means into a stream of gaspassing to the heating zone.

The mechanical means may comprise a screw feed. Alternatively themechanical means may comprise a rotatable cylindrical member provided onits cylindrical surface with a number of grooves to which powderedpolytetrafluoroethylene is fed and a duct for gas which crosses thesurface of the cylinder and is arranged so that a portion of thecylindrical member enters the duct and gas passing through the duct canpick up particles lodged in the grooves.

The invention also includes a feeding arrangement forpolytetrafluoroethylene particles, for use in the methods of the presentinvention, which arrangement comprises a rotatable cylindrical memberprovided on its cylindrical surface with a number of grooves, means forfeeding polytetrafluoroethylene powder to the surface of the cylindricalmember so that the grooves pick up powder when the cylindrical memberrotates, and a duct for conveying gas to the heating zone, which ductcrosses the cylindrical surface of the cylindrical member and is open topart of that surface so that gas passing through the duct can pick uppowder from any groove which cromes its path when the cylindrical memberrotates.

The invention also includes apparatus for use in spraying particles ofpolytetrafluoroethylene at or above sintering temperature in accordancewith the methods of the invention which apparatus comprises a tubularheating element, means at one end of the element for supporting a bed ofpolytetrafluoroethylene powder, means for directing a stream of gasthrough the tubular heating element and means for feedingpolytetrafluoroethylene powder into said stream of gas so thatpolytetrafluoroethylene particles are carried through the tubularheating element and out of the other end thereof when the apparatus isin use.

The invention further includes apparatus for use in spraying particlesof polytetrafluoroethylene at or above sintering temperature inaccordance with the methods of the invention which apparatus comprises atubular heating element, means at one end of the element for supportinga bed of polytetrafluoroethylene powder and means for directing a streamof gas (e.g. air) on to or through the bed to pick up particles and tocarry them through the tubular element and out of the other end thereof.

Preferably the tubular heating element comprises a source of infra-redradiation.

The invention still further includes apparatus for use in sprayingparticles of polytetrafluoroethylene at or above sintering temperaturein accordance with the methods of the invention which apparatuscomprises an elongated tube transparent to infra-red radiation, aninfra-red heating element surrounding said tube along a substantialproportion of its length, means for passing polytetrafluoroethyleneparticles through said elongated tube, whereby polytetrafluoroethyleneparticles may be brought to sintering temperature within said tube andprojected on to a surface to be coated.

The apparatus may comprise means for passing a stream of gas throughsaid elongated tube and mechanical means for feedingpolytetrafluoroethylene into said gas stream.

The mechanical means may comprise a rotatably cylindrical memberprovided on its cylindrical surface with a number of grooves, means forfeeding polytetrafluoroethylene powder to the surface of the cylindricalmember so that the grooves can pick up powder when the cylindricalmember rotates, and a duct for conveying gas to said elongated tube,which duct crosses the cylindrical surface of the cylindrical member andis open to part of that surface to that gas passing through the duct canpick up powder form any groove which crosses its path when thecylindrical member rotates.

The accompanying diagrammatic drawings illustrate various embodiments ofthe invention. In the drawings:

FIG. 1 is a sectional view of a spray gun in accordance with theinvention;

FIG. 2 is a sectional view of another spray gun together with a feedingarrangement therefor;

FIG. 3 is a perspective view of a feeding arrangement for use in thearrangement of FIG. 2;

FIG. 4 is a sectional view of another arrangement for feedingpolytetrafluoroethylene particles in accordance with the invention;

FIG. 5 is a sectional view of a valve for use in an arrangement of thekind shown in FIG. 4;

FIG. 6 is a sectional view of a further feeding arrangement inaccordance with the invention; and

FIG. 7 is a perspective view of a stirrer used in the arrangement shownin FIG. 6.

Referring to FIG. 1, the gun comprises a cylindrical casing l with aplatform 2 of 100 mesh gauze on which a bed 3 of drypolytetrafluoroethylene power may be supported. Beneath the platformthere is an inlet 4 for air by which the bed may be fluidized and someof the powder particles incorporated into the air stream. On the axis ofthe casing, above the bed, there is a quartz outlet element such as acylinder or tube 6 for the particle-carrying air, and a bafile 7, whichmay be adjustable towards and away from the mouth of the tube outletelement, to control the air flow. The cylinder or tube 6 passes throughan electrical heating element 8 and terminates in a noale outlet l0.

The heating element 8 may be of any suitable material. One of the itensfound to be effective is a silicon carbide element, cylinder or tube,preferably in one piece, in which the hot zone is in the form of a helixor some other equivalent shape. Obviously, many kinds and types ofelements of various shapes and sizes may be used One of the elements isthe kind sold by The Morgan-Crucible 00., Ltd. under their trade marCRUSILITE."

There is a lead-in cylinder tube 12 around the inlet end of the element8 with electrical connections to the element and surrounding the wholelength of the heating element there is a refractory cylinder or tube 14which, in turn, is surrounded by diatomaceous or other heat insulatingmaterial 15 in the space between the tube 14 and the casing. The ends ofthis space are closed by annuli 16, 17 of heat resisting material andthere is also an annular partition 18 supporting the platform 2 and thetube 6.

In use air is admitted through the inlet in an amount sufficient tofluidize the polytetrafluoroethylene powder and to entrain some of theparticles. The air passes out through the tube 6 and during its passagealong the tube the polytetrafluoroethylene particles are heated to alittle above their sintering temperature. The stream issuing from thenozzle 10 is directed on to a surface to be coated withpolytetrafluoroethylene, and as the hot particles strike the surface,they combine to form a substantially continuous film. shown).

A further embodiment of the invention will now be described withreference to FIG. 2 and 3. This apparatus comprises a quartz tube 20concentric with a second quartz tube 22 round which is helically wrappedan infrared heating element 23 connected to terminals 24, 25. A thirdquartz tube 26 surrounds the heating element and is provided on itsinner surface with a reflecting film of gold or rhodium. Cooling air orother gas enters the annular space between tubes 20 and 22 from inlet 40and leaves by an outlet (not shown). Tube 20 is connected to a feedingdevice which comprises a closed hopper 27 containing powderedpolytetrafluoroethylene 28. At the base of the hopper 27 there is arotatable cylinder 29 mounted on a shaft 30 driven by a motor 31. Thecylinder 29 has on its surface a plurality of helical grooves 32 whichpick up polytetrafluoroethylene powder from the hopper 27. A duct 33passes through the feeding device to the tube 20 and is open to part ofthe surface of the cylinder 29. Air or other gas blown through the duct33 picks up polytetrafluoroethylene powder from any groove which crossesthe path of duct 33 and conveys it to the tube 20.

In use the apparatus is placed with the nonle 21 of tube 20 directed atthe surface 34 which is to be coated. The hopper 27 is filled withpolytetrafluoroethylene powder and the motor 32 switched on to drive theshaft 30 and cylinder 29. The infra-red heating element 23 is switchedon previously to allow it to warm up. Nitrogen is blown through the duct33 and picks up polytetrafluoroethylene powder from the grooves 32 asportions of them cross the path of the nitrogen passing through theduct. The polytetrafluoroethylene powder then passes through tube 20where it is heated to sintering temperature or above by the infraredheating element. The hot particles are sprayed on to the surface 34where they coalesce to coat the surface. The infra-red radiation heatsup the polytetrafluoroethylene to a much greater extent than the carriergas.

The polytetrafluoroethylene particles may be preheated, e.g. to 250 C.in the feeding device.

Referring to FIG. 4, the arrangement comprises a container 50 having aporous plate 51 situated some way above its bottom. The container has ascrew top 52 through which two tubes 53 and 54 pass into the containerfrom a duct 55. Tube 53 passes through the porous plate 51 and is opento the space below it, whereas tube 54 opens to the space above theplate 51. A rod 56 passes through tube 53 and is connected to a valveplate 57 and screw 58. The screw 58 may be adjusted to vary the distancebetween the end of tube 53 and valve plate 57 and thus the amount of gaspassing into the container 50.

In use the gas for conveying polytetrafluoroethylene powder to theheating zone is passed through duct 55 and owing to a restrictioncomprising a throat 59in the duct some gas passes through tube 53 andenters the space below the porous plate 51. The gas passes through theporous plate 51 and fluidizes polytetrafluoroethylene particles storedin the container 50 above the plate. Some of these fluidized particlespass through tube 54 into duct 55 where they are conveyed to the heatingzone which may be in a tubular heating element as shown in FIGS. 1 or 2,or a plasma torch or gun of known kind.

FIG. 5 shows an alternative valve arrangement for adjusting the amountof gas flowing into tube 53 of the arrangement shown in FIG. 4. The tube53 is modified to provide a seating 60 for valve member 61 which can bescrewed down into tube 53 to adjust the amount of gas flowing through itinto container 50 for fluidizing the particles.

FIG. 6 shows a container 70 through which a duct 71 passes. A tube 72which leads into duct 71 contains a screw feed 73 on the end of arotatable rod 74 passing through the wall of the container 70. Anopening 75 is provided in tube 72 to admit powderedpolytetrafluoroethylene which is picked up by the screw feed 73 andconveyed to the duct 71. A stirrer 76 (FIG. 7) comprising a plate 77having a hole 79 through which rod 74 passes and four prongs 78 issituated in the container 70 to operate around the bottom of tube 72.

in use gas passing through duct 71 picks up powder introduced into theduct by screw feed 73 and conveys it to the heating zone.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Apparatus for coating a surface with a film ofpolytetrafluoroethylene comprising; a conduit made of a materialtransparent to infrared radiation for delivering a mixture ofpolytetrafluoroethylene particles to a surface to be coated through anexit noule, a cylindrical tube mounted around the conduit and made of amaterial transparent to infrared rays, an electrical resistance heaterpositioned around said tube for heating the particles by radiation to atemperature above the melting point of the particles, a reservoir forholding a quantity of said particles, said conduit connected to thereservoir at a position adjacent to the lower level of the particles, apipe having an exit port connected to the reservoir for forcing gasthrough the particles, and means in the reservoir for agitating theparticles.

2. Apparatus as set forth in claim 1 wherein the particles are heatedabove 327' C.

3. Apparatus as claimed in claim 1 wherein a rotatable cylindricalmember is mounted within said reservoir, said cylindrical member formedwith a helical groove on its outer surface, and a motor for turning thecylindrical member to agitate the particles.

4. Apparatus as claimed in claim 1 wherein said cylindrical tube is madeof fused quartz.

5. Apparatus as claimed in claim 1 wherein said constriction is placedin the pipe connected between the reservoir and the conduit and a branchpipe is connected to the conduit, said branch pipe extending into thereservoir and having an entrance port adjacent to the upper surface ofthe particles.

2. Apparatus as set forth in claim 1 wherein the particles are heatedabove 327* C.
 3. Apparatus as claimed in claim 1 wherein a rotatablecylindrical member is mounted within said reservoir, said cylindricalmember formed with a helical groove on its outer surface, and a motorfor turning the cylindrical member to agitate the particles. 4.Apparatus as claimed in claim 1 wherein said cylindrical tube is made offused quartz.
 5. Apparatus as claimed in claim 1 wherein saidconstriction is placed in the pipe connected between the reservoir andthe conduit and a branch pipe is connected to the conduit, said branchpipe extending into the reservoir and having an entrance port adjacentto the upper surface of the particles.